1. Field of Invention
The present invention relates to a subsea thermoelectric generator designed in particular for the equipment at drilling wellheads.
2. Description of Related Art
It is known that such a generator has semiconductor-type thermoelectric modules attached to a drilling line immersed in the sea water, in which line the petroleum product extracted from the well flows. This generator is capable of furnishing an electric current by the Seebeck effect due to the difference between the relatively high temperature (over 40.degree. C.) of the petroleum product extracted and the relatively low temperature (a few degrees) of the sea water.
The article "Subsea Thermoelectric Generators" by Jean-Pierre Von der Weid et al., published in Sea Technology, May 1994, pages 45-51, discloses a thermoelectric generator based on this principle. In this generator, which has cylindrical geometry, the thermoelectric modules are made of flat elements disposed concentrically in sleeves along a drilling line and connected electrically in series or in parallel. The surfaces of these modules are applied to the outside wall of the line to favor maximum transmission of heat flow between the fluid carried and the modules.
The modules are covered with rigid plates with high thermal conductivity, made of copper for example, to ensure good heat transmission between the modules and the sea water and to protect the modules against excessive compression forces manifested in the sea bed.
The peripheral edges of these plates rest on heat-insulating shoulders that compel the heat flow to pass principally through the thermoelectric modules. A resin seal is applied to the edges of the plates to prevent water from penetrating to the module.
However, these plates can deform under the force of the considerable hydrostatic pressure prevailing on the sea floor and thus apply compression forces to the thermoelectric modules that could destroy them. Moreover, since the resin seal is directly exposed to the sea water, local corrosion may cause it to become ineffective.
The article "Design of a Semiconductor Thermoelectric Generator for Remote Subsea Wellheads" by D. W. Auckland et al., published in IEEE Proc. Electr. Power Application, Volume 142, no. 2, March 1995, pages 65-69, discloses a thermoelectric generator provided with flat thermoelectric modules disposed concentrically along the line. The modules are attached to flat-bottomed pans machined into the outside wall of the line. To prevent short circuits between the modules, the latter are separated from the line and the walls of the pans by a layer of electrically insulating material. Likewise, the gaps between the modules are filled with a heat insulator which not only forces the heat flow principally through the modules but also prevents sea water from penetrating to the modules.
This generator suffers from the same defects as the generator described above because the layers of insulating material imperfectly protect the modules from outside compression forces.